The present invention relates to the technical field of steering systems and in particular to a steering system for a trailing axle of a vehicle.
Heavy vehicles—in particular commercial vehicles—often have more than two axles, so-called trailing axles. If the trailing axles are formed rigidly, the vehicles have a large turning circle. Therefore, in addition to front axle steering, a steerable trailing axle is often additionally incorporated. The trailing axle in this case can be positively steered or adhesively steered, i.e. steered by the restoring movement of the wheels themselves. This additional trailing axle steering permits smaller curve radii, which means that higher maneuverability is achieved. In addition, the slip angle on the tires is reduced, which means that the tire wear of the vehicle is reduced.
Active steering of the trailing axle is desirable only at low speeds, however. At higher vehicle speeds, no steering of the trailing axle is desired, since this has a detrimental influence on stable travel. The trailing axle must be fixed, starting at a specific speed dependent on the vehicle, in order not to cause any unstable traveling condition.
In such systems, it is advantageous that, in the event of a failure or at higher travel speeds, the axle can be kept in the straight-ahead position.
The prior art is that the trailing axle is steered via a hydraulic cylinder. The oil is pumped via a pump, which is driven via the internal combustion engine, into one or the other cylinder chamber, depending on how the valves are controlled. Therefore, in particular during straight-ahead travel, in which the vehicle remains for a long time, the hydraulic pump would be driven continuously although this is not necessary. In this operating state, the hydraulics generate losses, which are not countered by any added value. This is contrary to the requirement for a lower fuel consumption of the vehicle.
The problem is solved by the steering pump not being driven via the internal combustion engine but via an electric motor. Since the electric motor can drive equally in both directions, by using a reversible pump one or the other cylinder chamber can be pressurized, depending on the direction of rotation.
In German patent DE 4414161 C1, a multi-axle steering system is described in which a master cylinder is activated. Depending on the position of the master cylinder on the front axle, the slave cylinder on the rear axle reacts. The disadvantage with this system, however, is the direct dependence on the respective position of the master cylinder. Thus, with this system, there is no possibility of exerting a speed-dependent influence on the rear axle.
DE 103 51 482 A1 shows a steering system in which a hydraulic steered vehicle rear axle having an additional blocking device is kept in the current position or steered back into a central position by adhesion steering and is then locked. However, this requires further components, needs additional installation space and is thus expensive.
DE 10 2006 008 436 A1 shows a mechanically coupled multi-axle steering system in which a steering force is applied to the additional steering axle only when the latter is also required to be active—that is to say when there is a steering angle. However, this system can be implemented only with very great outlay for a rear axle steering system which is to be blocked in straight-ahead travel, starting from a certain speed range.
Finally, DE 10 2012 105 976 A1 discloses a steering system for a trailing axle having an electronic control system, in which the steering of the trailing axle takes place independently of the front axle. The pump is driven via an electric motor, which means that the system operates in an energy-efficient manner. The blocking function is implemented in the simplest way in that, during the adhesion-driven return movement of the piston, hydraulic fluid is discharged from the working cylinder through a central borehole. Once the piston reaches this central borehole, it closes the latter and therefore blocks any further movement.
Connected in the fluid connection between central borehole and pump is a valve which, in the working position, suppresses a fluid flow there. In the event of a fault, the valve is de-energized and then permits a fluid flow. This valve is implemented as a spool valve.
However, spool valves are susceptible to dirt. For example, metal particles floating about in the hydraulic fluid in the steering system can jam the piston.
The pump is connected to the two cylinder chambers of the working cylinder by a connecting line each. If the first cylinder chamber is filled with hydraulic fluid, the piston forces the hydraulic fluid out of the second cylinder chamber via the connecting line in the direction of the pump. The two connecting lines are therefore simultaneously feed and return line. The hydraulic fluid is practically always pushed to and fro in the circuit. As a result, a number of disadvantages arise, since air inclusions in the hydraulic fluid cannot be led out. In particular during the first filling, the system can be poorly vented. Furthermore, no filter can be incorporated in the circuit to filter dirt particles out of the hydraulic fluid, since the hydraulic fluid flows in both directions.